Sizing Circuit Breakers - TCC Curves - Arc Flash

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spikes2020

Member
Location
Nashville, TN
I recently did an arc flash study on one of our job sites, it was a standard 1500 kva transformer that had a normal 400A breaker coming off its buss. I was stunned when the results came back at 70 cal/cm^2. Looking at this TCC you can see that with a such a small arcing current that it took this breaker almost 50 seconds to trip.The red curve was the current settings. The blue fuse was added to protect the panel.

With so many breakers and sizes all with different tripping curves is there an easier way than going though them one by one? Here i had little over 2.5 kA arcing current. Such a low current didn't trip the breaker and thus caused the huge arc flash.

I am trying to select breakers from a catalog but i have to test each one's time curve to make sure it wont cause a high arc flash. Is there a better way?

Any Ideas?



 

skeshesh

Senior Member
Location
Los Angeles, Ca
I don't know of any other way than to be familiar with ocpd's by way of experience and keeping up to date with mfg literature. The breaker that was used was obviously not coordinated with the available fault current which is the case more often than not specially when dealing with something downstream in the LV distribution. One last thing to double check is your available fault, make sure it's not too low due to miscalculation.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
It looks like the magnetic trip point of the 400A breaker was set above the minimum level. Most 400A 480V breakers have magnetic trip settings below 2000A.
If your newly added fuse actually coordinates with the downstream equipment, it probably would have been easier to simply "dial down" the breaker.

In general, my experience has been, that up to 400A, breaker performance results in low arc flash incident energy.
 

spikes2020

Member
Location
Nashville, TN
It was a LHL36400 from Square D, its lowest trip setting was 2000A and the damn guys on site had it set for 4000A. But even at the 2000A the software said it wouldn't trip for a long time.

**Note the fault current was 2.13 ka via the breaker and i had another 90 amps from the down stream motors and the like. Then i took a + or - 15% on that for error.


Yeah, i was really trying to find a 400A lug to lug breaker that had a lower instant trip curve than the LHL36400 that i was using.
 

wiigelec

Member
Location
Red Desert
I am trying to select breakers from a catalog but i have to test each one's time curve to make sure it wont cause a high arc flash. Is there a better way?
Determine what TCC your application requires and then choose a breaker that meets those criteria...
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
That fault current looks extremely low for a 1500KVA transformer. Are you sure it is right? You must have a very long cable or something.

Also, unless you are in a manhole or a confined space, you can usually limit the calculation to a few seconds, and assume someone in an arc flash will move out of the way. SKM has an option to do that.

Steve
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
It was a LHL36400 from Square D, its lowest trip setting was 2000A and the damn guys on site had it set for 4000A. But even at the 2000A the software said it wouldn't trip for a long time.

**Note the fault current was 2.13 ka via the breaker and i had another 90 amps from the down stream motors and the like. Then i took a + or - 15% on that for error.


Yeah, i was really trying to find a 400A lug to lug breaker that had a lower instant trip curve than the LHL36400 that i was using.
You are correct, the LHL 400A breaker minimum setting is 2000A, which is to the center of the curve, your 15% tolerance would move you into the LTD portion of the curve. You may have to go to an electronic trip breaker for these low of currents (the LE equivalent to the LH goes down to 800A).
 

spikes2020

Member
Location
Nashville, TN
That fault current looks extremely low for a 1500KVA transformer. Are you sure it is right? You must have a very long cable or something.

Also, unless you are in a manhole or a confined space, you can usually limit the calculation to a few seconds, and assume someone in an arc flash will move out of the way. SKM has an option to do that.

Steve

Yeah there was about 1000 ft of 4/0 cable after this breaker...... this is for a construction site and is all temporary~ Really fun to design for...
 
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